Thermal relay

ABSTRACT

In a thermal relay of the type using a conductive heater layer to switch a layer of vanadium dioxide between nonconducting and conducting states, design advantages are attained by using a conductive substrate separated from the vanadium dioxide by a layer of insulative material having a thermal conductance per unit area that is within certain prescribed limits.

United States Patent inventors George E. Smith Relerenees Cited UNITED STATES PATENTS 1,63 l ,836 6/1927 Spray Primary Examiner-Rodney D. Bennett, Jr.

Assistant ExaminerR. Kinberg Attorneys-R. J. Guenther and Arthur J. Torsiglieri ABSTRACT: In a thermal relay of the type using a conductive heater layer to switch a layer of vanadium dioxide between nonconducting and conducting states, design advantages are attained by using a conductive substrate separated from the vanadium dioxide by a layer of insulative material having a thermal conductance per unit area that is within certain prescribed limits.

1 HQ 28 ,23 T33 WW/W/Wm 32 a v /25 THERMAL RELAY BACKGROUND OF THE INVENTION This invention relates to relays, and more particularly, to relays that make use of the negative temperature coefficient of resistivity of material such as vanadium dioxide.

The paper Thin-Film Switching Elements of V0," by K. van Steensel, F. van de Burg and C. Kooy, Philips Research Reports, Volume 22, pages 170-177, 1967, describes a fourterminal relay in which a conductive heater film is used to switch a layer of vanadium dioxide between conducting and nonconducting states. Vanadium dioxide is an example of a material which, because of its negative temperature coefiicient of resistivity, will act as a low resistivity metal if heated above a threshold temperature, but will act as a high resistivity semiconductor if maintained at a temperature below the threshold. The relay structure described by van Steensel et al. comprises a layer of vanadium dioxide on an insulative substrate such as glass, an insulative film such as silicon oxide overlaying the vanadium dioxide, and a conductive heater film overlaying the silicon oxide layer. By passing current through the heater layer, the vanadium dioxide is switched from a nonconducting to a conducting state in which it transmits current between opposite contacts.

It is of course desirable to optimize the operating parameters of the van Steensel et al. relay to conform to the systems in which it is to be used. In particular, it is ofien desirable to optimize the power required for switching and the time required for switching. These two parameters, however, are mutually dependent and one cannot be changed without changing the other.

SUMMARY OF THE INVENTION It is an object of this invention to provide a thermal relay in which the power required for switching and the time required for switching can be substantially independently designed.

This and other objects of the invention are attained in an illustrative embodiment thereof comprising a thermal relay of the general type described above. Rather than using an insulative substrate, however, a thermally conductive substrate is used which is separated from the vanadium dioxide layer by a thin layer of insulative material. The thermal conductance of the insulative layer is maintained within a specified range such that a thermal path is provided from the heater film through the vanadium dioxide layer to the conductive substrate. With this provision it can be shown that the power P required for switching and the time I required for switching can be op timized independently of each other. Since any desired switching time t and any desired switching power P can, within a rather broad range, be attained, the flexibility and number of uses to which the relay may be put are substantially increased.

These and other objects, features and advantages of the invention will be better understood from a consideration of the detailed description taken in conjunction with the accompanying drawing.

DRAWING DESCRIPTION FIG. 1 is a sectional view of a thermal relay of the prior art; and

FIG. 2 is a sectional view of a thermal relay in accordance with an illustrative embodiment of the invention.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown a thermal relay ll of the type described in the van Steensel et al. paper comprising a layer 12 of vanadium dioxide (V having contacts 14 and 15 on opposite ends and bonded to a substrate 13 of insulative material. A conductive heater film 17 having contacts 18 and 19 is located in close proximity to the V0 layer but is insulated from it by a layer 20 of insulative material.

When the V0, layer 12 is below a threshold temperature, typically 68 C., it acts as a semiconductor and displays a high resistivity to current between contacts 14 and 15. When the layer 12 is heated above its threshold temperature, due to heat generated by heater film 17, it acts as a low resistivity metal, and current flows freely between opposite contacts. Thus the structure of FIG. 1 constitutes a four-tenninal thermal relay: When current is directed through heater film 17, the tempera ture of the V0, layer 12 is raised above threshold and current is transmitted through a circuit including contacts 14 and 15; but, when current through the heater film 17 is terminated, current through the V0, layer is substantially reduced.

Of course, numerous four-terminal relay devices other than that shown in FIG. 1 are available to the engineer designing a system, and the one he chooses will significantly depend on the extent to which device parameters can be optimized to match the requirements of the system. Two parameters that characterize the device of FIG. 1 are the power P required by the heater film 17 to switch the V0, layer between conductive states, and the time t required for such switching. It can be shown that the switching power P of the FIG. 1 device is given by the equation P (1rlK'AT/2) (l) and the switching time t is given by t' (F/4a') (2) where K is the thermal conductivity of substrate 13, a is the thermal diffusivity of the substrate, AT is the difference in temperature between the transition temperature and the ambient temperature, and I is half the lateral dimension of the V0 layer 12 as shown in FIG. 1. Notice that only one geometrical parameter, namely the lateral dimension I, is contained in the expressions for P and 1. Hence, switching power and switching time cannot be independently determined in the design of the structure, and compromises between the two are inherent.

In accordance with the invention, this drawback is overcome by the relay 23 of FIG. 2 which uses a conductive substrate 24 rather than an insulative substrate. The vanadium dioxide layer 25 is separated from the conductive substrate by an insulative layer 26. A heater film 27 is separated from the V0 layer by an insulative layer 28. As before, heater current between contacts 30 and 31 is used to switch the states of the V0 layer 25, thereby controlling current between contacts 32 and 33. Since the substrate 24 is a good conductor of heat, a thermal path is provided that extends from heater film 27 to the substrate. This thermal path will transmit a significant quantity of heat compared to that transmitted by the contacts if the thermal conductance of insulative film 26 is within a specified range and if the combined thickness d of insulative layer 28, V0 layer 25 and insulative layer 26 is smaller than the lateral dimension 1. When this is true, it can be shown that the power P required for switching is given by P= (37rFKAT/8d) (3) and the switching time t is given by where K is the thermal conductivity of layer 26, and a is the thennal diffusivity of layer 26. Notice that in this case switching power P is a function of the lateral dimension I and the thickness dimension d while switching time t is a function only of the thickness dimension d shown in FIG. 2. This being true, switching time and switching power can be independently optimized by independent adjustment of the dimensions l and d.

Substrate 24 should of course be made of a material having high thermal conductivity such as copper. The insulative layer 26 may conveniently be silicon dioxide which has a thermal conductivity of l0 watts per centimeter-degree centigrade. Setting K equal to 10", table I shows various values of the thickness dimension d and the lateral dimension 1 that should be used for designing relays having switching powers of between 10' and 10" watts and switching times t of between 10 and 10" seconds, in accordance with equations (3) and (4):

TABLE l I (sec.)

4. I Hi Speed L Speed (#111) 10"" ows-.1

Lo Power P IO 0.6. 2.4 l7, 13 (watts) Hi Power The first number in each box refers to the thickness dimension d to be used and the second number to the lateral dimension 1. For example, if a high speed low power relay is designed to have a switching time t of seconds and a switching power P of 10"" watts, the thickness dimension d should be 0.6 microns and the lateral dimension 1 should be 2.4 microns.

A useful parameter for characterizing the insulative layer 26 is the conductance per unit area G which is related to the thermal conductivity K and thickness b by the equation 4 /b Assuming that K I0" watts per centimeter degree C., table I defines a range of values for 0,, assuming d =b, given y 53 6, 170 watts/cm-C. (6)

Materials having other values of thermal conductivity K could of course be used as the insulative film 26, but is is unlikely that a material having a thermal conductivity of substantially less than 10" or substantially more than 10 watts/cm C. would be useful in a thermal relay. Assuming that k is 10", the values of dimensions d and l are those given in table II:

TABLE II I (see) d, I Hi Speed Lo Speed (um) I0 l0" Lo Power P l0"" 0. I 7, 4.0 5.0. 20 (watts) Hi Power Table II leads to the following range of values of G 20 0, 560 watts/cm- C. (7) Assuming that K is 10"" watts/cm-C., the range of values for d and l is that given in table Ill.

TABLE III I (sec.)

d. I Hi Speed Lo Speed (pm) I0 10' Lo Power P IO'"'' 20. L6 60. 30 (watts) Hi Power cular configuration of the various layers of the structures of both FIGS. 1 and 2. To give a more general statement of the switching power P of the structure of FIG. 2 equation (3) may be expressed as P= CAKAT/d 10) where A is the area of V0 layer 25 and C is a constant dependent on device configuration which, in the case of a circular structure, is The constant is nearly equal to 56 for structures that are nearly symmetrical such as square configurations.

In summary, the criteria defining a thermal relay structure in which the switching power and switching time are mutually independent have been established. The ability to adjust switching power and switching time independently substantially increases the uses to which relays of this type may be put. For example, the relay of FIG. 2 may be used as an average power limiter in which a low power threshold and long switching time are simultaneously required. Moreover, it can be shown that if both the switching power P and the time t are sufiiciently small, the switching energy of the relay of FIG. 2 is substantially smaller than that of FIG. I. The ratio of switching energy e, of the relay of FIG. 2 to the switching energy e, of a comparable relay of FIG. 1 can be shown to be e le d]! (l 1) While the relay has been described as using a V0: layer. it is to be understood that any material having a negative temperature coefficient of resistivity could alternatively be used. Various other modifications and embodiments may be made by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A thermal relay comprising:

a layer of resistive material having a resistivity that varies as a function of temperature and having contacts at opposite ends thereof;

a conductive substrate on one side of the resistive layer and insulated therefrom by a first layer of insulative material;

a conductive heater layer on the opposite side of the resistive layer and insulated therefrom by a second layer of insulative material;

the first layer of insulative material having a thennal conductance per unit area G A which is less than approximately 500 watts per centimeter degree Centigrade.

2. The thermal relay of claim 1 wherein:

the layer ofresistive material has a negative coefficient of resistivity. 3. The thermal relay of claim 2 wherein: the parameter G in watts per centimeter degree centigrade substantially conforms to the relationship 2.0 G s 500.

4. The thermal relay of claim 3 wherein:

the thermal conductivity K of the first layer of insulative material is approximately 10' watts per centimeter degree centigrade and the parameter 6, substantially conforms to the relation 2050,5500.

5. The thermal relay of claim 3 wherein:

the thermal conductivity K of insulative material is approximately 10' watts per centimeter degree centigrade and the parameter G substantially conforms to the relationship 5.9SG Sl 70.

6. The thermal relay of claim 3 wherein:

the thermal conductivity K of the first layer of insulative material is approximately I0 watts per centimeter degree centigrade and the parameter G substantially conforms to the relationship 2.056 560.

7. The thermal relay of claim 3 wherein:

the first layer of insulative material is a silicon compound having thickness of between 0.17 and 60 microns.

8. The thermal relay of claim 7 wherein the first layer of insulative material is of silicon dioxide.

9. The thermal relay of claim 3 wherein:

the length of the layer of resistive material is larger than the combined thickness of the resistive material layer of insulative material and the second layer of insulative material.

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Patent No.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated November 16, 1971 line 69,

line 70,

line 73,

line 73, 7,

line 7 T,

TABLE I,

line

line line line

line 33,

TABIE II,

line

line 50,

under under Hi Speed,

under Lo Speed,

under Lo Power,

under Hi Power,

under under under Lo Power,

Hi Power,

should be should be should be should be should be should be should should be should be should be should be should be should be Inventor(s) George E Smith. and Robert H Walden Should should should should should should should should It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col.

be r

ORM F'O-IOSO 110-69) USCOMM-DC 60376-969 a U 5, GOVERNMENY PRINTING OFFICE Ifi 0-366-334 page 2 Patent No. 3,621,446 Dated November 97 In ent (s) George E Smith and Robert H. Walden It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

7306 6 Col. 3, TABLE III, under Hi Speed, 10 should be 10 Q. under Lo Speed, 10 3 should be 10 under Lo Power, lo' should be 1o' under H1 Power, 10' should be 10 001. a, line 3 of claim 4, 10' should be 10 line 3 or claim 5, lo should be 1o line 3 of claim 0, 1o' Should be 1o' line 3 of claim 9,, after- "layer" insert the first layer.

iigned en'i sealed this FEW-I day of May 1972.

(SE/IL) Atdxest.

EDWARD MFIFTCI-ER ,JP. ROBERT GOTTSCHALK AT; hes ting; Officer Commi ssioner' of Pa ten LS 

1. A thermal relay comprising: a layer of resistive material having a resistivity that varies as a function of temperature and having contacts at opposite ends thereof; a conductive substrate on one side of the resistive layer and insulated therefrom by a first layer of insulative material; a conductive heater layer on the opposite side of the resistive layer and insulated therefrom by a second layer of insulative material; the first layer of insulative material having a thermal conductance per unit area GA which is less than approximately 500 watts per centimeter2 - degree centigrade.
 2. The thermal relay of claim 1 wherein: the layer of resistive material has a negative coefficient of resistivity.
 3. The thermal relay of claim 2 wherein: the parameter GA in watts per centimeter2 - degree centigrade substantially conforms to the relationship 2.0 GA
 500. 4. The thermal relay of claim 3 wherein: the thermal conductivity K of the first layer of insulative material is approximately 10 1 watts per centimeter - degree centigrade and the parameter GA substantially conformS to the relation 20 GA
 500. 5. The thermal relay of claim 3 wherein: the thermal conductivity K of insulative material is approximately 10 2 watts per centimeter - degree centigrade and the parameter GA substantially conforms to the relationship 5.9 GA
 170. 6. The thermal relay of claim 3 wherein: the thermal conductivity K of the first layer of insulative material is approximately 10 3 watts per centimeter - degree centigrade and the parameter GA substantially conforms to the relationship 2.0 GA
 60. 7. The thermal relay of claim 3 wherein: the first layer of insulative material is a silicon compound having thickness of between 0.17 and 60 microns.
 8. The thermal relay of claim 7 wherein the first layer of insulative material is of silicon dioxide.
 9. The thermal relay of claim 3 wherein: the length of the layer of resistive material is larger than the combined thickness of the resistive material layer of insulative material and the second layer of insulative material. 